libdrm: Remove dependency towards X11

Using this defconfig

BR2_x86_pentium_mmx=y
BR2_ENABLE_DEBUG=y
BR2_OPTIMIZE_2=y
BR2_TOOLCHAIN_BUILDROOT_LARGEFILE=y
# BR2_UCLIBC_INSTALL_UTILS is not set
BR2_BINUTILS_VERSION_2_23_2=y
BR2_GCC_VERSION_4_8_X=y
BR2_PACKAGE_LIBDRM=y
BR2_PACKAGE_LIBDRM_RADEON=y
BR2_PACKAGE_LIBDRM_NOUVEAU=y
BR2_PACKAGE_LIBDRM_VMWGFX=y

libdrm (w/o Intel API) compiles without BR2_PACKAGE_LIBATOMIC_OPS and without
BR2_PACKAGE_XLIB_LIBPCIACCESS, so USE_WCHAR, TOOLCHAIN_HAS_THREADS and
!BR2_PREFER_STATIC_LIB, previously added by the Xorg dependency, do not seem
to be necessary for libdrm at all.

Signed-off-by: Bernd Kuhls <berndkuhls@hotmail.com>
Signed-Off-By: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Spenser Gilliland <spenser@gillilanding.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>